Non-chalking blowing agent for foamed polymers

ABSTRACT

A method of making a foamed polymer article by injection molding, using a foaming agent that does not form byproducts which could pit or chalk the polymer surface. The foaming agent contains sodium borohydride in an amount from 0.01 to 5.0 weight percent of the polymer, and sodium dihydrogen phosphate as an activator to react with the sodium borohydride and produce hydrogen gas. The sodium borohydride and the sodium dihydrogen phosphate are kept separated from direct contact with each other, such as by encapsulating the components, until gas production is desired when the polymer is heated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to foamed polymer products. Moreparticularly, the invention concerns a chemical foaming agent and amethod of using such agent to make molded foamed polymer articles.

2. Description of Related Art

Foaming agents, also known as blowing agents, are used during processingof polymers to achieve a cellular structure in the final product. Asdescribed in the article "Foaming Agents" in Modern PlasticsEncyclopedia (Mid-October 1989 Issue Volume 66, Number 11), chemicalblowing agents (CBAs) are substances that decompose at elevatedtemperatures or through chemical reaction and form gaseous compositionproducts. Reasons given for the use of foaming agents include weightreduction for savings in cost and material, better properties such asinsulation against heat or noise, different surface appearances,improved stiffness, and different electrical properties.

One widely used inorganic chemical blowing agent is sodium borohydride("SBH"). As explained in Lapierre et al. U.S. Pat. No. 4,769,397, SBHreacts with water or some other proton donor or "activator" to releasegas for foaming. That patent describes an activation system comprising amixture of sodium bicarbonate and citric acid which reacted to evolvewater and carbon dioxide.

In some applications the use of a sodium bicarbonate/citric acidactivation system has been associated with a phenomenon known as"chalking," in which a white material believed to be borax and sodiumcarbonate forms on the surface of a molded part. One object of thisinvention is to provide an improved activator for SBH to avoid chalkingproblems.

Commercially available foaming agents are typically in the form ofconcentrates: active ingredients encapsulated in a carrier resin.Another object of this invention is to provide an improved foaming agentconcentrate which is stable during storage.

SUMMARY OF THE INVENTION

The invention concerns a foamable polymeric composition comprising apolymer to be foamed and a foaming agent. The agent contains an alkalimetal borohydride such as sodium borohydride in an amount from 0.01 to5.0 weight percent of the polymer, and an amount of sodium dihydrogenphosphate sufficient to react with the sodium borohydride and producegas. The sodium borohydride and the sodium dihydrogen phosphate are keptisolated from direct contact with each other, such as by encapsulatingthe components, until gas production is desired.

DETAILED DESCRIPTION OF THE INVENTION

Novel foaming agents and methods for making foamed injection-moldedarticles using such compositions are described below. In the method ofthe invention, a foaming agent comprising SBH, an activator for the SBH,and a polymer to be foamed are all fed to an injection molding machine.The polymer melts as it moves through the machine's barrel and is mixedwith the SBH and activator. The SBH and activator react in this mixtureand evolve gas, resulting in a foamed polymer article.

Sodium Borohydride

Sodium borohydride ("SBH") is available from Morton International ofDanvers, Massachusetts. To protect the SBH from contact with moisture orfrom other conditions which would cause a premature reaction, the SBH ispreferably encapsulated in a carrier resin.

Borohydrides of other alkali metals such as lithium, potassium,rubidium, and cesium should be considered equivalents and can be used inthis invention, although sodium borohydride is preferred.

Sodium Dihydrogen Phosphate

Although SBH can react with many possible proton donor activators toevolve a hydrogen gas, the choice of a suitable activator for use infoamed polymer products is more limited.

Not only must the activator function at the intended polymer processingconditions, but it should have no undesirable characteristics such ashandling attributes, effects on processing equipment, or appearance inthe final foamed product. An activator ideally should be non-volatileunder the reaction conditions encountered during mixing and molding andshould not form corrosive byproducts in reaction with the sodiumborohydride. Both the activator and its reaction products with SBHshould be dispersible in the polymer to be foamed.

Unexpectedly it has been found that sodium dihydrogen phosphate willfunction well as an activator for SBH. Sodium dihydrogen phosphate,sometimes referred to as monobasic sodium phosphate, sodium acidphosphate, monobasic sodium orthophosphate, or sodium biphosphate, is awhite powder that is stable until reacted with the SBH. This offers anadvantage over dual-component activator systems such as sodiumbicarbonate/citric acid in which the activator components may contacteach other during encapsulation and can evolve water prematurely.

Foaming Agent

The foaming agent comprises SBH and the activator. The amount of theagent used to foam a polymer will depend upon the desired density of thefoamed product, i.e., the amount of foaming desired. Generally theconcentration of SBH is low, ranging from 0.01 to 5.0, preferably 0.1 to3.0, most preferably 0.1 to 0.3 percent by weight of the polymer to befamed. Too little SBH and/or insufficient activator will produceinsufficient foaming, while excess SBH will cause collapse of the cellsin the foamed structure.

The amount of activator should be sufficient to react with the SBH andthereby cause at least some foaming of the polymer. In determining therelative amounts of activator and SBH in the blowing agent composition,the activator is advantageously available in at least stoichiometricamounts based upon the proton donor content of the activator. Thereaction is expected to proceed according to the following formula:

    NaBH.sub.4 +2NaH.sub.2 PO.sub.4 H.sub.2 O→4H.sub.2 +Na.sub.2 H.sub.2 P.sub.2 O.sub.7 +NaBO.sub.2 +H.sub.2 O

Anhydrous sodium dihydrogen phosphate may also be used, at greaterexpense. The molar ratio of sodium dihydrogen phosphate to SBH can rangewidely but is preferably to 1:1 to 4:1, more preferably about 2:1to2.5:1. A slight excess amount of activator is preferred, primarily foreconomic reasons, to maximize the potential of the relatively expensiveSBH.

The SBH and the activator should be separated from direct contact witheach other until gas production is desired. Separation of thesecomponents can be accomplished by known means. For example, theindividual components can each be fed to the machine by separate hoppersor feed systems so that they contact each other only inside the extruderbarrel. Encapsulation of one or both of the foaming agent components ina carrier resin is preferred. The encapsulated components can then bepremixed.

The encapsulating or carrier resin may be polystyrene, polyethylene,polypropylene or any other resin that is compatible with the processingconditions of the polymer to be foamed and that does not adverselyaffect the desired characteristics of the foamed article. Ideally thecarrier resin is the same material as the polymer to be foamed.

Encapsulated SBH is commercially available from Quantum ChemicalCorporation, USI Division, for example FM1776H which is 10% by weightSBH in a polystyrene carrier. The sodium dihydrogen phosphate isconveniently encapsulated by melt-blending with a polyolefin and thenpelletizing to form a concentrate which preferably consists of up to50%, more preferably about 40%, by weight of the activator.

Method of Foaming Resin

The foaming agent of this invention can be used in almost anythermoplastic resin. Among the more commonly foamed resins are ethyleneand propylene homopolymers and copolymers, polystyrene,acrylonitrile-butadiene-styrene (ABS), poly(phenylene oxide) (PPO), andpoly(phenylene ether) (PPE).

The amount of foaming agent relative to the polymer to be foamed ispreferably large enough to be easily and precisely measured, but smallenough so that the agent (and particularly the carrier resin) willconstitute a small percentage of the polymer. The exact amount is opento choice, but generally about 0.1-3.0 parts by weight of the foamingagent (consisting of encapsulated SBH and the encapsulated activator)are present per 100 parts by weight of the resin to be foamed. About 2parts per hundred are preferred.

In the method of making a foamed injection-molded article according tothe invention, the foaming agent is dispersed into polymer such as byconventional dry blending. The polymer and foaming agent mixture isheated to a molten state whereby the foaming agent components react andevolve gas. The mixture is subsequently injected into a mold where itexpands and cools into a molded foamed article.

EXAMPLE

Sodium borohydride concentrate FM1776H was obtained from QuantumChemical Corporation, USI Division. This pellet form concentrate wasprepared by melt compounding with polystyrene and consists of 10 percentby weight SBH.

The activator was dispersed in a carrier resin by the followingprocedure. About 2.5 lbs. of PETROTHENE NA279 low density polyethylene(LDPE) from Quantum Chemical Corporation was milled on a two-roll millat 280° F. until it formed a continuous crepe. An equal amount of sodiumdihydrogen phosphate was gradually added to the crepe at the nip of thetwo rolls. The composition was milled for five minutes, at which timethe sodium dihydrogen phosphate had dispersed in the LDPE matrix. Thecrepe was removed from the mill and cooled to room temperature and thenground to a powder.

A mixture of 380 grams of the polystyrene-encapsulated SBH and 552 gramsof the polyethylene-encapsulated activator was prepared by tumbleblending. This mixture was then tumble blended with 2134 grams of impactgrade polystyrene. The blend was then injection molded into specimens at425° F. to 440° F. using an extruder.

The molded samples exhibited 28.6% lower density than the correspondingunfoamed polystyrene under identical conditions, 0.75 versus 1.05 g/cc.No pitting or chalking was noticed after several weeks storage atambient conditions.

We claim:
 1. A method of making a foamed injection-molded article, themethod comprising:dispersing into a polymer a foaming agent comprising(i) sodium borohydride and (ii) sodium dihydrogen phosphate to form amixture, with the sodium borohydride comprising from 0.01 to 5.0 weightpercent of the mixture; heating the mixture whereby the foaming agentreleases gas; and subsequently injecting the mixture into a moldobtaining expansion of said polymer into a molded foamed article.
 2. Themethod of claim 1 in which the foaming agent contains from one to fourmoles sodium dihydrogen phosphate per mole of sodium borohydride.
 3. Themethod of claim 2 in which the foaming agent contains from 2 to 2.5moles sodium dihydrogen phosphate per mole of sodium borohydride.
 4. Themethod of claim 1 in which the sodium borohydride and sodium dihydrogenphosphate are separately encapsulated in a carrier resin prior to beingdispersed into the polymer.
 5. The method of claim 4 in which thecarrier resin is selected from the group consisting of polystyrene,polyethylene, and polypropylene.
 6. The method of claim 4 in which thesodium borohydride comprises about 10 weight percent of theresin-encapsulated sodium borohydride.
 7. The method of claim 4 in whichthe sodium dihydrogen phosphate comprises from 40 to 50 weight percentof the resin-encapsulated sodium dihydrogen phosphate.
 8. The method ofclaim 1 in which the polymer is selected from the group consisting ofethylene and propylene homopolymers and copolymers, polystyrene,acrylonitrile-butadiene-styrene, poly(phenylene oxide), andpoly(phenylene ether).